齿 is the literary/formal word for tooth. In everyday speech, 牙 (yá) or 牙齿 (yáchǐ) is used. 齿 appears in many compounds: 齿轮 (gear), 口齿 (articulation), 不齿 (to despise).
history
The traditional character 齒 is a pictograph resembling a mouth with teeth. It's one of the 214 Kangxi radicals (radical 211).
